1. Field of the Invention
The present invention relates to a hydraulic pump unit having a plurality of pump elements, disposed in one plane, each of which has one pump cylinder and one pump piston longitudinally displaceable in the pump cylinder. The longitudinal axes of the pump elements intersect at a common intersection point. The hydraulic pump unit has a cam drive with cams, which upon a rotation of the cam drive relative to the pump elements act upon the pump pistons with an actuating motion. The number and the angular spacing of the cams is adapted to the number and angular spacing of the pump elements in such a way that when a certain number of pump pistons are located at top dead center, the same number of pump pistons are at bottom dead center.
The invention also relates to a high-pressure pump for generating an injection pressure for internal combustion engines.
2. Prior Art
The hydraulic pump units of the type defined at the outset are also known as radial piston pumps. They are used predominantly as high-pressure pumps for generating an injection fuel pressure for an internal combustion engine. However, other possible uses of the hydraulic pump units are also conceivable.
From the prior art, hydraulic pump units of the type defined at the outset are known in various versions. For instance, there are hydraulic pump units in which two pump elements are disposed in an angular spacing of 180xc2x0 from one another. The pump elements of this known hydraulic pump unit are actuated from inside by a camshaft, which extends perpendicular to the plane of the pump elements and through the intersection point of the longitudinal axes of the pump elements. On the outer circumference of the camshaft, two cams are disposed at an angular spacing of 180xc2x0; upon a rotation of the camshaft relative to the pump elements, these cams act upon the pump pistons with an actuating motion. In this internally actuated hydraulic pump unit, the pump pistons are pressed radially inward by spring elements and are pressed outward by the cams of the camshaft, counter to the force of the spring elements. Since the pump elements and the cams are disposed at an angular spacing of 180xc2x0 from one another, the pump elements are both actuated simultaneously by the cams of the camshaft. The pumping courses of the pump elements are superimposed on one another. Both pump pistons are simultaneously in either a pumping motion or an intake motion. Both pump pistons reach top dead center or bottom dead center at the same instant. The result is a highly uneven pumping characteristic of the known hydraulic pump unit.
In the prior art, externally actuated hydraulic pump units with two or four pump elements, disposed at an angular spacing of 180xc2x0 and 90xc2x0, respectively, from one another, are also known. These units for instance have a cam ring, on whose inside circumference two cams at an angular spacing of 180xc2x0, or four cams at an angular spacing of 90xc2x0, are embodied. By rotating the cam ring relative to the pump elements, the pump pistons are acted upon by the cams with an actuating motion. In these externally actuated hydraulic pump units, the pump pistons are pressed radially outward by spring elements and are pressed radially inward by the cams of the camshaft counter to the force of the spring elements. In this known embodiment as well, the pump pistons of all the pump elements are in either a pumping motion or an intake motion simultaneously, and all the pump pistons reach either the top dead center or the bottom dead center, as applicable, at the same instant. The result in these known hydraulic pump units as well is a highly uneven pumping characteristic.
Finally, another embodiment of a hydraulic pump unit known from the prior art can be mentioned, in which three pump elements are disposed at an angular spacing of 120xc2x0 from one another. The pump elements are actuated from inside via a camshaft, on whose outer circumference two cams are disposed at an angular spacing of 180xc2x0 from one another. The pumping courses of the pump elements of this known hydraulic pump assembly are shown in FIG. 5. The pump pistons of the pump elements C, D, E are located partly in a pumping motion (in the upper region 90) and in an intake motion (in the lower region 91) at the same time, resulting in overlaps 92 in the pumping courses and overlaps 93 in the intake courses of the individual pump elements C, D, E. Because of the overlaps 92, 93, the result in this known hydraulic pump unit is again an uneven pumping characteristic, especially if the pump elements C, D, E are only partly filled.
From the European and German references EP 0 517 991 A1, DE 31 13 737 A1 and DE 1 503 356 A, hydraulic pump units of the type defined at the outset are known. However, these known hydraulic pump units have so many pump elements that a plurality of pump elements are simultaneously in an intake phase or a pumping phase, and thus the pumping courses of the pump elements have overlapping ranges. In intake-regulated hydraulic pump units, this can lead to unequal filling of the pump elements during the intake phase and a resultant unequal pumping during the pumping phase, since the pump units, for technical production reasons, are not embodied absolutely identically. This has an adverse effect on the power and noise production in the hydraulic pump unit. As prior art, reference is also made to U.S. Pat. Nos. 2,423,701 and 5,701,873.
It is the object of the present invention for a hydraulic pump unit of the type defined at the outset to be designed and refined in such a way that particularly in the event of partial filling, it makes uniform pumping of the hydraulic fluid possible.
To attain this object, based on the hydraulic pump unit of the type defined at the outset, the invention proposes that the cam drive has precisely two cams, disposed at an angular spacing of 180xc2x0 from one another, and the hydraulic pump unit has precisely two pump elements disposed at an angular spacing of 90xc2x0 from one another.
In a further way of attaining the object of the present invention it is proposed that the cam drive has precisely three cams, disposed at an angular spacing of 120xc2x0 from one another, and the hydraulic pump unit has precisely two pump elements disposed at an angular spacing of 60xc2x0 from one another.
According to the invention, it has been recognized that the hydraulic pump unit has an especially uniform pumping characteristic whenever the hydraulic pump unit has an even number of pump elements, so that in each case an equal number of pump elements can be actuated contrary to one another, and at any instant the same equal number of pump pistons are located in opposed pumping and intake positions.
In the hydraulic pump unit of the invention, a certain number of pump pistons are acted upon, upon a rotation of the cam drive relative to the pump elements, with a pumping motion or an intake motion. At the same time the same number of pump pistons is acted upon by an opposite actuating motion in the form of an intake motion and a pumping motion, respectively. For instance if the instant of the actuating motion at which a certain number of the pump pistons is at top dead center is discovered, then at the same instant the same number of pump pistons is at bottom dead center.
The hydraulic pump unit of the invention, especially in the event of partial filling of the pump elements with fuel, has an especially uniform pumping characteristic, since in the pumping courses of the individual pump elements of the hydraulic pump unit, overlaps or, which would be worse, superpositions do not occur.
In a preferred refinement of the present invention, it is proposed that the cams of the cam drive act upon the pump pistons from inside with an actuating motion. Advantageously, the cam drive is embodied as a camshaft, which extends through the intersection point, perpendicular to the plane in which the pump elements are located, and on whose outer circumferential surface the cams are embodied.
In another advantageous refinement of the present invention, the pump pistons pump the hydraulic fluid to the radially outer end of the pump elements. From the radially outer ends of the pump elements, the pumped hydraulic fluid can then be carried out of the hydraulic pump unit and delivered to a common rail via supply lines located outside the hydraulic pump unit. As a result, the high pressure is shifted out of the housing of the hydraulic pump unit into the externally located supply lines. The housing can be designed for merely low pressure and can be produced correspondingly less expensively.
Finally, it is proposed that the hydraulic pump unit be used as a high-pressure pump for generating an injection pressure for a fuel-operated internal combustion engine. Especially in a use as a high-pressure pump, the advantages of the hydraulic pump unit of the invention, that is, a uniform pumping characteristic and a high degree of operating smoothness, become especially important.
As another way of attaining the object of the present invention, it is proposed on the basis of the high-pressure pump of the type defined at the outset that a plurality of the hydraulic pump units of the invention are disposed one after the other in such a way that the pump elements of all the hydraulic pump units are acted upon with an actuating motion by the same cam drive.
In an advantageous refinement of the present invention, it is proposed that two hydraulic pump units are disposed one after the other in such a way that the planes in which the pump units are located extend parallel to one another, and that the intersection points of the pump units of the hydraulic pump units are located on the same camshaft.
Advantageously, the hydraulic pump units are disposed congruently one after the other. Alternatively, it is proposed that the hydraulic pump units are disposed one after the other, rotated by an angular offset of 180xc2x0.